The 28th International Cosmic Ray Conference

1The Telescope Array Experiment: Hybrid Measurement of Ultra HighEnergy Cosmic Rays in Northern Hemisphere
Fumio Kakimoto1 , Shoichi Ogio1 , Ryuichi Minakawa1 , Masaki Fukushima2 ,Nobuyuki Sakurai2 , Naokai Hayashida2 , Hideaki Shimodaira2 , Reiko Torii2 ,Masato Takita2 , Yasuo Arai3 , Takeshi Matsuda3 , Hirofumi Fujii3 , Manabu
Tanaka3 , Yoshito Tanaka4 and TA group(1) Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan(2) Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 2778582, Japan(3) Institute of Particle and Nuclear studies, KEK, Tsukuba 305-0801, Japan(4) Nagasaki Institute of Applied Science, Nagasaki 851-01, JapanAbstract
The result of AGASA shows the extension of the cosmic ray energy spectrum above the GZK cut-oﬀ. Several new generation experiments are plannedor under construction to identify the origin of such ultra high energy (UHE) cosmic rays exceeding the cut-oﬀ. We report here the design of hybrid Telescope
Array (TA) and introduce plans of improving the accuracy of measuring the energy spectrum by the hybrid measurement of ground array and air ﬂuorescencetelescope.
1.

Introduction

The AGASA energy spectrum shows that there is no indication of GZK
cut-oﬀ expected by the photo-pion production of the ultra high energy cosmic
rays. There is also an indication of point sources in the arrival direction of suchcosmic rays [1]. If the origin of these cosmic rays is some type of already known
astronomical object, it must exist within several tens of Mpc from our galaxy.
Until now no astronomical object is identiﬁed in the directions of the observed
UHE cosmic rays. Several top-down models are proposed as a remedy, in which
UHE cosmic rays are produced via anomalous physics processes or by unknown
astronomical objects [2]. In order to get a deﬁnite answer on the origin of UHE
cosmic rays, it is crucially important to measure the energy spectrum, the arrival
distribution and the composition more accurately and compare them with thepredictions of these models. We therefore proposed the Telescope Array as thenext generation air shower experiment [3] and planned to observe air showers by
the air ﬂuorescence technique. On the other hand, HiRes group recently reported
that there is a GZK cut-oﬀ in their observed energy spectrum [4]. It seems clear
pp. 1–4c2003
by Universal Academy Press, Inc.

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that a part of the inconsistency is due to the systematic error of both experimentsin the determination of primary cosmic ray energies [4]. Taking this criticalsituation into account, we now intend to observe air showers with an AGASA typeground array and with an air ﬂuorescence detector simultaneously as the ﬁrst stepof the TA project. In this paper, we present a simulation study to conﬁrm how
much the systematic error is improved in the primary energy estimation by using
our hybrid detector.
2.

Experimental Plan

In our design of thehybrid-TA, an array of 24×24
plastic scintillators covering theground area of 760 km2 issurrounded by three air ﬂuorescence stations. The West
Desert in Utah, USA is taken as
1. The prototype camera and telescope reflector forthe experimental site. The to- Fig.thefluorescence measurement.
tal ﬁeld of view of each stationis 3◦ − 34◦ in the elevation angle and 120◦ in the azimuthal angle, with all of
the three stations looking toward the center of the ground array. The separationof the station is 30 − 40 km and the stereo acceptance is ∼670 km2 sr for UHE
cosmic rays falling within 45 km of the station. The ﬂuorescence acceptance alone
is 4 times larger than that of AGASA assuming 10% duty factor. Approximately
60% of the events for E > 1020 eV is observed by all 3 stations.
The ﬁeld of view of each telescope is 18.0◦ in azimuth and 15.5◦ in elevation.
The telescope has a spherical dish of 3.3 m diameter composed of 18 hexagonal
segment mirrors (see Fig.1). The focal length is 2960 mm and the spot size on thefocal plane is ∼30 mm in FWHM according to a ray tracing calculation. The air
shower image is recorded by a camera composed of 16×16 2-inch PMTs placed on
the imaging plane (see Fig.1). Each PMT pixel covers 1.1◦ ×1.0◦ patch of the sky.
A signal from the PMT is ampliﬁed by a factor of 4 with a pre-ampliﬁer and issent to a Charge Successive Integrators (CSI) circuit. The CSI integrates a signal
every 200 ns and send it out successively to a 12-bit pipeline ADC by means of 3
capacitances operated in a rotational sequence. A self-ranging H/L gain scheme
is adopted to cover a dynamic range corresponding to 1−64k photoelectrons.
The digitized signal is fed to a Digital Signal Processor (DSP) to search for a
ﬂuorescence signal by maximizing the S/N ratio in 25.6 µs time window.
By one year operation of the hybrid-TA, we will collect ∼12 events with
E > 1020 eV and more than 900 events with E > 1019 eV, of which ∼80 eventsare measured simultaneously by both the scintillation detector array and the air
ﬂuorescence telescopes.

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Simulation Procedure

The simulation of UHE cosmic rays by the full Monte-Carlo is practicallyprohibited by the computation time. In our analysis of the hybrid event, weﬁrst generate UHE events using a thinning method. The proton is assumed as aprimary and the ﬁrst interaction depths are sampled by the Monte Carlo simulation. The longitudinal development (LD) proﬁles are calculated analytically witha modiﬁed Gaisser-Hillas function, in which 6 parameters characterizing LDs areestimated with empirical formulas. These formulas are derived from the results ofa small amount of simulated events with Corsika Monte Carlo package. Fluorescence and Cherenkov photons are generated according to the LD function and thetransmission of photons in the atmosphere is simulated taking eﬀects of Rayleighand Mie scattering into account. For the determination of the primary energy,
it is important to estimate the eﬀects of these scatterings accurately. Note thatin the event generation we adopted 1.2 km for the aerosol scale height, h, and
20 km for the attenuation length for the Mie scattering. In the ﬁnal stage of thesimulation, PMT output signals are generated by taking optical properties of thetelescopes and cameras into account. The produced shower signal was analyzed,and its energy and arrival direction were reconstructed by the χ2 minimization
method. Details of the analysis procedure are described in our design report [3].
4.

Method to Improve Systematic Error

Following is one of the ways we are considering to improve the systematic
error of the energy determination using the hybrid event. (a) First a primaryenergy, E, and an air shower size at the array altitude, NF , are estimated witha corresponding LD proﬁle reconstructed by the air ﬂuorescence measurement.
(b) Then the obtained E is corrected by multiplying a factor NG /NF , where NG
is the measured shower size by the ground array. In the AGASA experiment,
the primary energy is estimated from the observed local particle density at thecore distance of 600m. In this procedure, a major part of 18% total systematic
error comes from the uncertainty in the adopted Monte Carlo simulation notably
from the diﬀerences of used hadronic interaction models and the assumed primary
composition. On the other hand, the measurement of NG itself can be as goodas 10% or better, and this is used for the correction of E in our method. In thefollowing analysis, we artiﬁcially produced ∼20% systematic error in the ﬂuorescence energy measurement by reconstructing the LD proﬁle using a“wrong scaleheight of 2.0 km and 0.4 km against 1.2 km used in the event generation. Whereas
the value of NG at the array altitude is generated by smearing the correct shower
size by 10%.

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ResultThe simulation is performed for primary energies of 1019 , 1020 and 1021 eVwith zenith angles less than 60◦ and with impact parameters from the center of theground array less than 10 km. For each primary energy, 200 events are generated.
The result for the events with E = 1020 eV
is shown in Figure 2. Before the correction, the estimated energies are ∼14% lessor ∼21% more than the generated value of1020 eV by adopting the wrong scale heightof 0.4 km or 2 km. This is shown in theupper panels of Figure 2. After the correction, the systematic shift of the measured
energy is reduced as seen in the lower twopanels of the Figure 2. The present results
are summarized in Table 1. It is shown
Fig. 2. Distributions of the energy measure- that even when there is ∼20% systematic
ment errors for E = 1020 eV by using a shift in the air ﬂuorescence energy measure“wrong scale height of 0.4 km (a and c)
ment, we can reduce the error within ∼10%and 2 km (b and d).
by adopting our method with a minor sacriﬁce in the resolution. Now we are improving our reconstruction program in order to reduce the systematic error evenfurther.
Table 1.

The improvement of the systematic error.

EAdopted h av. E shift ± resol.
(eV)
(km)
before correction
1019
0.4−9.5 ± 6.0%
20
100.4
−13.9 ± 6.3%
21100.4−17.7 ± 6.8%
1910
2.0+21.7 ± 6.0%
2010
2.0+21.2 ± 4.8%21102.0+21.8 ± 5.8%

av. E shift ± resol.
after correction+14.9 ± 16.7%+10.1 ± 18.4%+8.6 ± 17.1%−4.1 ± 12.5%+4.8 ± 13.1%+2.9 ± 11.7%

References[1] Takeda, M. et al., 1998, Phys. Rev. Lett. 81, 1163.
[2] For example, P. Bhattacharjee et al., 1992, Phys. Rev. Lett. 69, 567.
[3] The Telescope Array Project: Design Report July 2000.
[4] T. Abu-Zayyad et al., 2002, astro-ph0208310 and astro-ph0208243